Transition in Force Balance during Puysegur Subduction Initiation: Three-dimensional Gravity and Dynamic Models

Abstract:

Subduction initiation remains an unsolved problem for plate tectonics, but recent seismic images of the structure and history of the nascent Puysegur Trench, south of New Zealand, provide key constraints on this phenomenon. This system is a young subduction zone with a well constrained tectonic history and is currently transitioning from a forced to a self-sustaining state. We model the subsurface density structure of the Puysegur subduction zone and the Fiordland block onshore using a joint free-air-Bouguer 3D linear gravity inversion constrained by seismic, bathymetric, and geologic data within a Bayesian framework. The density model and residual topography imply that the system is out of isostatic equilibrium and that the changes in topographic height along Puysegur Ridge and up into Fiordland cannot be explained by density variations alone; these changes must be dynamically forced. Using the geodynamic modeling code Underworld, we have developed models of the changing force balance during subduction initiation that are directly constrained by a combination of gravity, seismic, and bathymetric data from the Puysegur system. This density structure is mapped to a 3-D finite element mesh, setting the a priori geologic structure and buoyancy forces. With Stokes flow and a viscoelastic rheology, we obtain the force balance and viscosity structure that best reproduces the observed topographic changes along and across the Puysegur margin. Dynamic subsidence of the Snares Zone, which is a locus of strike-slip deformation predominantly underlain by buoyant continental crust, is likely the result of the lateral propagation of elastic stresses within the slab as its negative buoyancy continues to develop beneath Fiordland. The propagation of these stresses allows for the development of a trench parallel gravity low at the Snares Zone. In contrast, high effective topography at the southern end of the Puysegur Ridge is the result of dynamic uplift as the slab begins to be forced under the ridge by continued convergence. We also model the dynamic contribution to the Fiordland gravity high. These bathymetric and topographic changes along the system from north to south and the forces required to sustain them illustrate the transition in force balance taking place over the course of subduction initiation.